Research On Mechanism Of Desulfurization And Denitrification By Dielectric Barrier Discharge Combined With Wet Scrubbing Under Oxygen Enriched Condition

Ocean going ships undertake more than 90%of the global freight transportation,but also cause serious air pollution.In order to control the exhaust pollution of marine diesel engine,the International Maritime Organization（IMO）regulated the emission of NO_x and SO_x from marine diesel engine exhaust.By comparing various NO_x and SO_x control technologies,it is considered that low temperature plasma combined with wet scrubbing desulfurization and denitrification technology has the advantages of fast reaction rate and can operate at low exhaust temperature,which is a promising technology.Dielectric barrier discharge（DBD）has the characteristics of stable and uniform discharge,which is the most promising low-temperature plasma technology for industrial application.But at present,the energy consumption of low temperature plasma equipment is high,and the mechanism of desulfurization and denitrification of low temperature plasma under oxygen enriched diesel engine is less studied.In order to improve the oxidation efficiency of DBD and reduce energy consumption,an integrated system of DBD combined with wet scrubbing desulfurization and denitrification was built,and the desulfurization and denitrification mechanism was studied by using emission spectroscopy（OES）and numerical calculation.（1）Firstly,the effects of O₂,H₂O,CO₂ and C₃H₆（hydrocarbon）on the oxidative removal of NO and SO₂ in DBD reactor were studied under the mixture of NO/SO₂/CO₂/C₃H₆/H₂O/O₂/N₂.In the range of 0%-15%,O₂ significantly promoted the formation of O radicals,but the promotion of OH radicals was limited.When O₂ increased from 0%to 15%,NO removal efficiency increased from 8.5%to 54.2%,and about 41.3%of the removed NO was converted to NO₂.When O₂ increases from 0%to 5%,the removal efficiency of SO₂ increases from 8.7%to 24.8%;If O₂ continues to increase to 15%,the removal efficiency of SO₂ will slow down to 23.5%due to SO+O+M→SO+O+M.In the range of 0%-9.8%,with the increase of H₂O,the oxidation removal efficiency of NO increases from 18.9%to 57.3%（about 19%of which is oxidized to NO₂）,the removal efficiency of SO₂ increases from 4.8%to 35.3%,and the CO decreases from 460×10 to 229×10.However,the effect of H₂O decreases with the increase of H₂O content.The results of OES and numerical calculation show that the increase of H₂O content promotes the formation of OH radical and consumes O radical（O（D）+H O→2OH）.The electronegativity of H₂O（adsorbing electrons）leads to the decrease of electron density,and the increase of OH radical production will slow down with the increase of H₂O content.In the range of 0%-10.5%,the concentration of CO₂ has little effect on the formation of O and OH radicals.Therefore,increasing the concentration of CO₂ has little effect on the oxidative removal of NO and SO₂,but it will increase the production of CO.In the range of 0-1 000×10,the increase of C₃H₆ increased the NO oxidation removal efficiency from 42.8%to 70.5%,SO₂ removal efficiency from 26.9%to 32.5%,but at the same time increased CO from 178×10 to 530×10.The results of OES showed that C₃H₆ consumed O and OH radicals,and the generated HO₂ and RO₂replaced O and OH and became the main substances for NO oxidation.OH is the main species of SO₂ removal.Therefore,C₃H₆ has limited effect on SO₂ removal.（2）Secondly,the effects of power supply parameters and DBD structure parameters on NO and SO₂ oxidation were studied in this paper under the simple component（NO_x/O₂/N₂）and complex component（NO/SO₂/CO₂/C₃H₆/H₂O/O₂/N₂）.The oxidation efficiency of DBD reactor to NO is basically the same as the same energy density at different voltage and frequency.The oxidation degree of NO_xincreases first and then decreases with the increase of energy density.The high electron energy will lead to N₂ decomposition to produce N free radicals,and NO will be produced by side reactions,which will eventually lead to the reduction of NO_xoxidation degree.The increase of energy density will promote the removal of SO₂,but also promote CO generation.The reaction time is insufficient due to the short electrode;Too long electrode will lead to the increase of energy consumption in the reactor.Compared with 50 mm and150 mm,DBD reactor has the highest efficiency in NO oxidation and SO₂ removal when the electrode length is 100 mm.Decreasing E/N can promote the formation of N（X,v）,increasing E/N can promote the formation of N.The DBD reactor with electrode diameter of 10 mm is difficult to inhibit the formation of N（X,v）and N at the same time,so it is easier to produce NO_x and the oxidation efficiency of NO is low.The increase of electrode diameter to 14 mm makes DBD reactor more easy to operate under the high O and OH radical generation efficiency,avoids side reactions and reduces the energy consumption.OES showed that the temperature of DBD reactor with smaller electrode diameter was higher,which increased the oxidation rate of SO₂.Therefore,although the efficiency of O,OH and O₃ generation is low,the removal of SO₂ is not affected.The electric field strength on the surface of the screw electrode is much higher than that of the rod electrode,which is more likely to produce N-free radicals to produce NO.Therefore,the oxidation removal efficiency of NO is lower and the energy consumption is higher.The results of OES and numerical calculation show that the OH radical generation efficiency of DBD reactor with screw electrode is equivalent to that of rod electrode,but the gas temperature in discharge gap is higher.Therefore,the removal efficiency of SO₂ by rod electrode is lower than that of screw electrode.Under the simple components,the stainless steel electrode has the best oxidation efficiency on NO when the diameter of the electrode is 10 mm.When the electrode diameter is 14 mm,the oxidation efficiency of copper rod on NO is better.The discharge gap has obvious interaction with electrode materials.Under the complex components,the different stainless steel electrodes（304,316L and 310S）have no effect on the NO oxidation,SO₂ removal efficiency and CO generation.（3）Finally,DBD reactor combined wet scrubbing tower were used to carry out desulfurization and denitrification test,and analyze the influence weight of various factors by orthogonal test.The results show that the desulfurization efficiency can easily reach more than 97%,and denitration is the key problem.The influence weights of DBD discharge power,simulated flue gas flow,NaOH solution mass fraction,spray density,filler type and height on denitration efficiency are 56.96%,18.02%,11.52%,5.02%,4.33%and 4.16%respectively.